The present invention relates to a hydraulic motor provided with two cylinder sets with diameters of different sizes and with a sliding spindle valve structure, controlled by the pressure medium being arranged in a bore. This bore, which is formed in the motor shaft, guides the pressure medium alternatively or simultaneously into the larger-diameter cylinder set or into the smaller-diameter cylinder set. In known motors of this kind, the pistons of the cylinder set remaining without pressure medium are guided out of contact with a cam ring whereby the pistons do not participate in the rotation of the motor. This arrangement suffers from the disadvantage that, when reconnecting the deactivated pistons or, for example, when shifting from one gear to another, powerful pressure impulses are produced along with the sudden changes in volume. When the volume of the motor increases while shifting gear, or when the disconnected pistons are thrust into operation, the pistons are unable to engage the cam ring quickly enough. The roller bearings in the ends of the piston hit against the cams of the rotating cam ring, causing impacts in the cam ring and in the bearing. These impacts, especially at higher speeds, may cause damage and shorten the lifetime of the motor.
Due to the above-mentioned circumstances, in a coupling system of this type in which the disconnection takes place in the manner described above for some or all the pistons of the motor, the motor can be run at relatively low speeds of revolution only, and the system can be used in slow vehicles or as auxiliary power when moving at slow speeds, while being disconnected when driving at high speeds.